Making timer reset to 45:00 minutes?

I'm using a timer which counts up on a button click when i press it, and when it reaches the time i desire, usually around the 45 minute mark i click my other button which stops the timer and the time remains static, so if i click my button to stop and my label says "45:34" then it will just stay like that as a static label. This is fine, i have done all this with no problem. The problem i have though is being able to resume this timer but start it from "45:00" instead of "00:00"